1. Field of the Invention
The present invention relates to a quantum cascade laser.
2. Description of the Related Art
A quantum cascade laser (QCL) emits light of a mid-infrared wavelength range of approximately 3 μm to 20 μm. (A quantum cascade laser may hereunder be referred to as “QCL”.) Light of the mid-infrared wavelength range is used in environmental gas analysis, medical diagnosis, and industrial processing. Therefore, a quantum cascade laser is used in these fields. In particular, a quantum well cascade laser is small, has high-speed characteristics, and is low in cost. Therefore, the quantum well cascade laser is promising as a light source of the mid-infrared wavelength range.
In conventional QCLs, in order to achieve laser oscillation, it is necessary to supply high electric power of a few W to a core region, which is a light emitting layer. Therefore, when driving the QCLs, a large amount of heat is generated in the core region. This heat causes the temperature of the core region to rise excessively. As a result, QCL characteristics are deteriorated; for example, threshold current is increased, light output is reduced, and high-temperature operation becomes difficult to achieve.
Recently, a QCL including a multi-core structure (divided-core structure) in which a core region of the QCL is divided into a plurality of tiny regions is proposed. Such a QCL is described in, for example, “Applied Physics Letters, vol. 101, 041113, 2012” (Non Patent Literature 1 (NPL 1)) and “AIP ADVANCES, 1, 032165, 2011” (Non Patent Literature 2 (NPL 2)).
For example, the structure that is described in NPL 2 is schematically shown in FIG. 16. As shown in FIG. 16, in a QCL having a divided-core structure, buried regions having a width D are formed between many core regions (such as 16 regions) having a uniform width W. The buried regions are formed of InP. InP is a semiconductor material providing good thermal conductivity among semiconductor materials usable in QCLs of the mid-infrared wavelength range. Further, similarly to the buried regions, an upper layer at an upper side of the buried regions and a lower layer at a lower side of the buried regions are also formed of InP.
By such a divided-core structure, heat generated at the core regions is transmitted in a direction that is parallel to a principal surface of a substrate and perpendicular to a waveguide direction (hereunder referred to as “transverse direction”) via the buried regions, and is efficiently dissipated to the outside of the core regions. In other words, by such a divided-core structure, heat dissipation in the transverse direction at each core region is improved.
As a result, compared to conventional QCLs in which core regions are not divided, the QCL having the divided-core structure is capable of sufficiently suppressing a temperature rise of each core region during operating. Therefore, QCL characteristics are improved; for example, light output is increased and maximum operating temperature is enhanced. NPL 1 actually reports that the QCL having the divided-core structure has a smaller thermal resistance, as compared to conventional QCLs in which core regions are not divided.